1. Field of the Invention
The field of the present invention pertains to the field of power supplies and among other things to the regulation of power supplies.
2. Background of the Invention
Power supplies that convert an AC mains voltage to a DC voltage for use by integrated electronic devices, amongst other devices, are known. The power supplies are required to maintain the output voltage, current or power within a regulated range for efficient and safe operation of the electronic device. Switches that operate according a pulse width modulated control to maintain the output voltage, current, or power of the power supply within a regulated range are also known. These switches utilize an oscillator and related circuitry to vary the switching frequency of operation of the switch, and therefore regulated the power, current or voltage that is supplied by the power supply.
A problem with utilizing pulse width modulated switches is that they operate at a relatively high frequency compared to the frequency of the AC mains voltage, which results in a high frequency signal being generated by the power supply. This high frequency signal is injected back into the AC mains input and becomes a component of the AC mains signal. The high frequency signals are also radiated by the power supply as electromagnetic waves. These high frequency signals add to the Electromagnetic Interference (EMI) of the power supply, and in fact are the largest contributors to the EMI of the power supply. The EMI generated by the power supply can cause problems for communications devices in the vicinity of the power supply and the high frequency signal which becomes a component of the AC mains signal will be provided to other devices in the power grid which also causes noise problems for those devices. Further, the radiated EMI by the power supply can interfere with radio and television transmissions that are transmitted over the air by various entities.
To combat the problem of EMI, several specifications have been developed by the Federal Communications Commission (FCC) in the United States and the European Community (EC) have established specification that specify the maximum amount of EMI that can be produced by classes of electronic devices. Since power supplies generate a major component of the EMI for electronic devices, an important step in designing a power supply is minimizing the EMI provided by the power supply to levels with the acceptable limits of the various standards. Since, a power supply can be utilized in many different countries of the world, the EMI produced should be within the most stringent limits worldwide to allow for maximum utilization of the power supply.
A known way of minimizing the EMI provided by the power supply is by adding an EMI filter to the input of the power supply. An EMI filter generally utilizes at least one inductor, capacitor and resistor in combination. However, the greater EMI produced by the power supply the larger the components that are utilized as part of the EMI filter. The cost of the EMI filter is in large part determined by the size of the inductor and capacitor utilized. The longer the components, the higher the cost of the power supply. Further, simply utilizing an EMI filter does not address the radiated EMI.
Another problem associated with pulse width modulated switches results from operation of the power supply at start up. At start up, the voltage, current and power at the output of the power supply will essentially be zero. The pulse width modulated switch will then conduct for the maximum possible amount of time in each cycle of operation. The result of this is a maximum inrush current into the power supply. The maximum inrush current is greater than the current that is utilized during normal operation of the power supply. The maximum inrush current stresses the components of power supply and switch. Stress is specifically a problem for the switch, or transistor, the transformer of the power supply, and the secondary side components of the power supply. The stress caused by the maximum inrush current decreases the overall life of the power supply and increases the cost of the power supply because the maximum rating of the components used in the power supply to not destruct from the inrush currents will be greater than the maximum rating required for normal operation.
Further, when the pulse width modulated switch conducts for the maximum possible amount of time in each cycle of operation the voltage, current and power at the output of the power supply rise rapidly. Since the feedback circuit of the power supply often does not respond as fast as the operating frequency of the switch, the rapid rise of the voltage, current and power will often result in an overshoot of the maximum voltage in the regulation range which will cause damage to the device being supplied power by the power supply.
Referring to FIG. 1 a known power supply that attempts to minimize EMI and reduce startup stress is depicted. A rectifier 10 rectifies the filtered AC mains voltage 5, from EMI filter 120, input by the AC mains to generate a rectified voltage 15. Power supply capacitor 20 then generates a substantially DC voltage with a ripple component. The rectified voltage 15 with ripple component is provided to the primary winding 35 of transformer 40 that is used to provide power to secondary winding 45. The output of secondary winding 45 is provided to secondary rectifier 50 and secondary capacitor 55 that provide a secondary DC voltage 60 at the power supply output 65 to the device that is coupled to the power supply.
In order to maintain the secondary DC voltage within a regulate range a feedback loop including an optocoupler 70, zener diode 75 and a feedback resistor 80 provides a signal indicative of the voltage at the power supply output 65 to feedback pin 85 of pulse width modulated switch 90. The voltage magnitude at the feedback terminal is utilized to vary the duty cycle of a switch coupled between the drain terminal 95 and common terminal 100 of the pulse width modulated switch 90. By varying the duty cycle of the switch the average current flowing through the primary winding and therefore the energy stored by the transformer 40 which in turn controls the power supplied to the power supply output 65 is kept within the regulated range. A compensation circuit 105 is coupled to the feedback pin 85 in order to lower the bandwidth of the frequency of operation of the pulse width modulator.
Inrush currents are minimized at start up by use of soft start capacitor 110. Soft start functionality is termed to be a functionality that reduces the inrush currents at start up. At this instant a current begins to flow through feedback resistor 80 and thereby into soft start capacitor 110. As the voltage of soft start capacitor 110 increases slowly, current will flow through light emitting diode 115 of optocoupler 70 thereby controlling the duty cycle of the switch. Once the voltage of the soft start capacitor 110 reaches the reverse breakdown voltage of zener diode 75 current will flow through zener diode 75. The approach described above will reduce the inrush currents into the power supply, however, it will be several cycles before the light emitting diode 115 will begin conducting. During the several cycles the maximum inrush current will still flow through the primary winding and other secondary side components. During these cycles the transformer may saturate, and therefore the transformer may have to be designed utilizing a higher core size than would be required for normal operation even with the use of soft start capacitor as in FIG. 1.
To reduce the EMI output by the power supply an EMI filter 120 is utilized. Additionally, pulse width modulated switch 90 is equipped with frequency oscillation terminals 125 and 130. Frequency oscillation terminal 125 and 130 receive a jitter current 135 that varies according to the ripple component of substantially DC voltage 15. The jitter current 135 is used to vary the frequency of the saw-toothed waveform generated by the oscillator contained in the pulse width modulated switch 90. The saw toothed waveform generated by the oscillator is compared to the feedback provided at the feedback pin 85. As the frequency of the saw toothed waveform varies, so will the switching frequency of the switch coupled between the drain and common terminal. This allows the switching frequency of the switch to be spread over a larger bandwidth, which minimizes the peak value of the EMI generated by the power supply at each frequency. By reducing the EMI the ability to comply with government standards is increased, because the government standards specify quasi-peak and average values at given frequency levels. Varying the frequency of operation of the pulse width modulated switch by varying the oscillation frequency of the oscillator is referred to as frequency jitter.
A problem associated with the EMI reduction scheme described with respect to FIG. 1 is that the ripple component will have variances due to variations in the line voltage and output load. Additionally, since the ripple may vary, design and the component value of EMI resistor 140 is difficult to determine and correspondingly design of the power supply becomes problematic.